1. Field of the Invention
This invention relates to microporous membranes and more particularly to cationic charge modified microporous membranes suitable for the filtration of high purity water as used in the electonics industry.
2. Prior Art
Microporous membranes are well known in the art. For example, U.S. Pat. No. 3,876,738 to Marinaccio et al (1975) describes a process for preparing a microporous membrane, for example, by quenching a solution of a film forming polymer in a non-solvent system for the polymer. European patent application No. 0 005 536 to Pall (1979) describes a similar process to Marinaccio et al.
Other processes for producing microporous membranes are described for example, in the following patents:
U.S. Pat. No. 2,783,894 to Lovell et al;
U.S. Pat. No. 3,642,668 to Bailey et al;
U.S. Pat. No. 4,203,847 to Grandine, II;
U.S. Pat. No. 4,203,848 to Grandine, II; and
U.S. Pat. No. 4,247,498 to Castro.
Attempts have also been made to charge modify microporous membranes. For example, U.S. Pat. No. 4,125,462 to Latty (1978) describes a coated membrane having a layer or coating of a cationic polyelectrolyte, preferably poly(vinylimidazoline) in the bisulfate form. Other types of charge modified membranes are described in, for example, U.S. Pat. No. 3,556,992 to Massucco; U.S. Pat. No. 3,556,305 to Shorr; U.S. Pat. No. 3,808,305 to Gregor; and U.S. Pat. No. 4,250,029 to Kiset et al.
In the assignee's U.S. Ser. No. 201,366 filed Oct. 27, 1980 by Ostreicher et al now abandoned, and copending U.S. Ser. No. 314,307 filed on Oct. 23, 1981 (which is a continuation-in-part of U.S. Ser. No. 201,366), a particularly preferred charge modified microporous membrane is described. The membrane, preferably nylon, comprises a multiplicity of cationic charge sites on the internal pore surfaces. The charge sites are provided by a cationic charge modifying resin, in particular a polyamido-polyamine epichlorohydrin resin, bonded to the membrane structure. The membrane is further provided with a cross-linking agent for the charge modifying resin which is effective in retaining the resin on the membrane. A preferred cross-linking agent is a polyamine, e.g. tetraethylene pentamine. Such a membrane is sold under the Trademark ZETAPOR by AMF Cuno, Meriden, Conn.
The primary advantage of a charge modified microporous membrane is the exclusion of particulate on the basis of charge as well as size; thus, the example, viruses can be removed from a fluid without having to go to an ultrafiltration membrane with its associated high pressures.
In the production of solid state electronic devices, e.g. computer chips, it is critical that any water utilized in the production process be of the highest possible purity in terms of freedom from both dissolved and undissolved contaminants. Normally, filtered 18 megohm-cm resistivity water is used in such applications. Such water is generally produced by activated carbon and ion exchange deep bed treatments (to remove dissolved organic and inorganic contaminants) followed by filtration (to remove small particulate and undissolved contaminants). Given the practical cost and particle size removal limitations of conventional mechanical barrier type filters, vis-a-vis charge modified filters, the present filtration systems are not completely satisfactory in terms of removal of colloidal contaminants, waterborne bacteria, etc. Additionally, without exception, all of the known filtration media seriously reduce the resistivity of the deionized water being filtered by introducing a significant amount of inorganic ionic contamination to the water. While conventional polymeric membrane filters which are not charge modified suffer from this defect to some degree, they tend to flush out very rapidly, e.g. within 5 to 30 minutes. Such a flush out procedure is a normal part of the water system start up after changing a filter. Such mechanical barrier type membrane filters remove small particulate and undissolved contaminants, by providing a sufficiently small effective pore dimension to remove, by mechanical straining, the undesired contaminants. Such filter structure, in the form of microporous membranes of, for example, 0.1 micrometer rating or less, may be readily prepared. Such membranes tend to flush out very rapidly, however, the flow rates exhibited by such structures at conventional pressure drops are prohibitively limited. Increasing the pressure drop through the membrane to provide the desired flow rate is generally not feasible, even with costly replacement or modification of existing pumping equipment. This is due to the fact that pressure drop is a function of the fourth power of flow rate.
While it would appear advantageous to use a cationically charge modified filter media, e.g. a microporous membrane, to provide for the economical and effective removal of undissolved particulate contaminants, current state of the art media exhibit certain characteristics that prevent their use in such applications. Currently available charge modified microporous media, for example, the aforementioned ZETAPOR cationically modified nylon membrane prepared in accordance with the aforementioned U.S. Ser. No. 201,366, exhibit an extremely slow flush out characteristic, i.e. up to several hours. This characteristic has prevented the use of such charge modified media in such high purity electronic water systems.